1. Field of the Invention
The present invention relates to a variable-speed V-belt drive for a vehicle with an engine.
2. Description of Related Art
Referring to FIG. 13 showing a related art of a variable-speed V-belt drive for a vehicle with an engine disclosed in JP-A 173392/1999, a driven pulley 200 has a fixed sheave 201 and a movable sheave 202, and a cylindrical cam shaft 203 having helical cam grooves 211. The cylindrical cam shaft 203 is formed integrally with the fixed sheave 201 of the driven pulley 200 and is fixedly mounted on a driven shaft 207. A sleeve 205 formed integrally with the movable sheave 202 is mounted on the cylindrical cam shaft 203 so that the sleeve 205 can axially move and rotate.
A pressure adjusting mechanism for the driven pulley 200 includes the cylindrical cam shaft 203, the sleeve 205, rollers 214 retained on the sleeve 205 and engaged in the cam grooves 211 of the cylindrical cam shaft 203, and pressure springs 206 biasing the movable sheave 202 toward the fixed sheave 201.
Referring to FIG. 14 showing the cylindrical cam shaft 203 and the sleeve 205 in an exploded perspective view, each cam groove 211 has a first working surface 211a and a second working surface 211b opposite and parallel to the first working surface 211a. The first working surface 211a, which is on the front side with respect to a rotating direction F, of each cam groove 211 engages with the roller 214 when the engine is operating in a driving mode. The second working surface 211b, which is on the rear side with respect to the rotating direction F, of each cam groove 211 engages with the roller 214 when the engine is operating in an engine-braking mode. The working surfaces 211a and 211b are smooth, continuous surfaces from one end of the cam groove 211 for a high reduction ratio to the other end of the same for a low reduction ratio to enable the rollers to move smoothly along the cam groove 211.
When the vehicle travels down a hill in an engine-braking mode, the movable sheave 202 of the driven pulley 200 is shifted to a position for a high reduction ratio by the pressure springs 206 and the each roller 214 is at a position near the position for a maximum reduction ratio in the cam groove 211 (FIG. 14) to set the variable V-belt drive at a high reduction ratio.
As shown in FIG. 14, the second working surface 211b of the cam groove 211, with which the roller 214 engages while the vehicle is traveling in the engine-braking mode, is parallel to the first working surface 211a and is a smooth, continuous surface. Therefore, it is possible that the roller 214 is forced to move from a position near the position for the maximum reduction ratio along the second working surface 211b toward a position for a low reduction ratio when the vehicle travels down a hill of a steep slop in the range of about 20xc2x0 to about 30xc2x0 and the rotative driving force of the rear wheels of the vehicle is very high. Consequently, the effective pitch diameter of the driven pulley 200 is decreased and the effect of engine brake is reduced.
Accordingly, it is an object of the present invention to provide a variable-speed V-belt drive for a vehicle with an engine, capable of maintaining a high engine-braking effect even while the vehicle is traveling down a hill, of enabling moderate shift from a normal engine-driving mode to an engine-braking mode without causing the vehicle to generate shocks when the vehicle starts traveling down a hill, and of enabling smooth shift from the engine-braking mode to the normal engine-driving mode.
According to a first aspect of the present invention, a variable-speed V-belt drive for a vehicle with an engine, comprises: a drive pulley mounted on a drive shaft; a thrusting mechanism mounted on the drive shaft and capable of exerting thrust to the drive pulley to vary effective pitch diameter of the drive pulley; a driven pulley mounted on a driven shaft, the driven pulley including a fixed sheave fixedly mounted on the driven shaft and a movable sheave, the fixed sheave being integrally provided with a cylindrical shaft, the movable sheave being mounted on the cylindrical shaft so that the movable sheave can rotate and axially move on the cylindrical shaft; a V belt extended between the drive pulley and the driven pulley; and a pressure mechanism of applying pressure to the movable sheave of the driven pulley, the pressure mechanism including a helical cam groove formed in one of the cylindrical shaft and the movable sheave of the driven pulley, and a roller mounted on other of the cylindrical shaft and the movable sheave of the driven pulley so that the roller engages with the cam groove, the cam groove having opposite side surfaces respectively serving as a first working surface with which the roller engages when the vehicle travels in an engine-driving mode and a second working surface with which the roller engages when the vehicle travels in an engine-braking mode, the second working surface of the cam groove having a retaining surface of retaining the roller thereon when the vehicle travels in the engine-braking mode.
Even when the vehicle travels down a hill in the engine-braking mode and the rotative driving force of the rear wheels of the vehicle increases, the roller is retained on the retaining surface of the second working surface of the cam groove to maintain the variable-speed V-belt drive at a fixed reduction ratio. Consequently, the high effect of engine brake can be maintained.
Preferably, the retaining surface is formed so that the roller is retained at a position displaced from a position for a maximum reduction ratio toward a position for a low reduction ratio.
Thus, the engine-driving mode is changed moderately to the engine-braking mode when the vehicle starts traveling down a hill.
Preferably, the second working surface is provided with a recess involving the retaining surface and having a depth equal to about half a diameter of the roller.
Thus, the recess involving the retaining surface retains the roller therein to maintain the effect of engine-braking, and enables the roller to move out of the recess smoothly when the traveling mode of the vehicle changes from the engine-braking mode to the engine-driving mode to ensure smooth change from the engine-braking mode to the engine-driving mode.
Preferably, the recess involves the retaining surface and a guide surface, the guide surface extending from the retaining surface and smoothly merging into an end surface of the cam groove on a side of the position for the maximum reduction ratio.
Thus, the traveling mode of the vehicle can be smoothly changed from the engine-driving mode to the engine-braking mode when the vehicle starts traveling down a hill.
According to a second aspect of the present invention, a variable-speed V-belt drive for a vehicle with an engine, comprises: a drive pulley mounted on a drive shaft; a thrusting mechanism mounted on the drive shaft and capable of exerting thrust to the drive pulley to vary effective pitch diameter of the drive pulley; a driven pulley mounted on a driven shaft, the driven pulley including a fixed sheave fixedly mounted on the driven shaft and a movable sheave, the fixed sheave being integrally provided with a cylindrical shaft, the movable sheave being mounted on the cylindrical shaft so that the movable sheave can rotate and axially move on the cylindrical shaft; a V belt extended between the drive pulley and the driven pulley; and a pressure mechanism of applying pressure to the movable sheave of the driven pulley, the pressure mechanism including a helical cam groove formed in one of the cylindrical shaft and the movable sheave of the driven pulley, and a roller mounted on other of the cylindrical shaft and the movable sheave of the driven pulley so that the roller engages with the cam groove, the cam groove having opposite side surfaces respectively serving as a first working surface with which the roller engages when the vehicle travels in an engine-driving mode and a second working surface with which the roller engages when the vehicle travels in an engine-braking mode, the second working surface of the cam groove including a segment having a large lead angle which is larger than a lead angle of the first working surface.
Even when the vehicle travels down a hill in the engine-braking mode and the rotative driving force of the rear wheels of the vehicle increases, the section having a large lead angle restrains the roller from moving toward a position for a low reduction ratio, prevents excessively high effect of engine braking, and prevents the reduction of the effect of engine brake moderately.
Preferably, the segment having the large lead angle extends from a position, which is axially displaced from a position for a maximum reduction ratio in the cam groove toward a position for a low reduction ratio, toward the position for the low reduction ratio.
Thus, the sharp increase of the effect of engine brake can be prevented.
Preferably, the segment having the large lead angle extends from a position, which axially coincides with a position for a maximum reduction ratio in the cam groove, toward a position for a low reduction ratios.
Thus, the reduction of the effect of engine brake can be prevented and engine brake becomes effective quickly.